This is a good book to read if you want to understand the motivation behind Cohen's forcing technique used to create models of set theory in which various propositions (such as the axiom of choice and the continuum hypothesis) can be forced to be true or forced to be false. When I read it I was struck by the use of Martin's axiom to motivate forcing, which works very well. You should read Paul Cohen's great monograph "Set Theory and the Continuum Hypothesis" of course, in which (in book form) forcing originated; but forcing developed rapidly in the 1960s and 70s, and Kunen's book captures the developments until the 1980s. It is a great achievement, and does a brilliant job of categorising the different approaches to forcing. I would also recommend Smullyan and Fitting's "Set Theory and the Continuum Problem", which introduces modal models (ie models expressed in the logic of possibility and necessity) as a way of understanding forcing. I gave it 4 stars only because a lot has happened with forcing since the 1980s; but before reading Jech's "Set Theory" you should read Kunen's book.